How to Determine why Plastic Parts Fail

Manufacturers using plastic parts in their products and processes are routinely required to analyze failed parts to determine the root cause of the failure and the appropriate corrective actions.

This analysis is typically performed using infrared and Raman spectroscopy to evaluate the parts’ chemical composition, UV-visible spectroscopy to investigate their optical transmissivity and color and thermal analysis to determine their physical properties.

This article outlines a study that employed all of these tools to ascertain the cause of a plastic part’s failure.

Thermo Scientific Nicolet iS50 FTIR Spectrometer.

Thermo Scientific Nicolet iS50 FTIR Spectrometer. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

Case Study

A company manufacturing precision optical equipment had designed a plastic cover for a device. This part was designed to possess specific characteristics in terms of its chemical composition, surface texture, color and optical transmission.

The cover was manufactured from a polycarbonate–acrylonitrile butadiene styrene (PC-ABS) blend. It also included enough titanium dioxide to deliver a subtle off-white color and ensure optical transmissivity of less than 0.01% T over a wide spectral range – ranging from UV into near-infrared.

The part’s opacity was necessary to inhibit ambient light entering the optical device and adversely affecting low light level measurements. All parts supplied were initially found to be within specification, and product performance was satisfactory.

A re-engineering project was implemented to lower costs and make the product more competitive. This process involved the commissioning of various parts – including the cover - from alternate suppliers.

A new supplier quoted lower than the original cover supplier, and test parts were found to meet all opacity requirements, so the production contract for that part was awarded to the new supplier.

The product started to fail critical performance tests shortly after this change, with failures traced back to the presence of ambient light causing elevated backgrounds, impacting low level optical measurements.

A visual inspection did not highlight any observable differences from the original part, but a number of control experiments confirmed that the failure was a result of the new cover.

It was, therefore, necessary to undertake a root cause analysis using a range of applicable techniques to rapidly identify and contain the issue.

Experimental Results

UV-Visible Spectroscopy

Diffuse transmission measurements were taken from both the original cover and failed cover using a Thermo Scientific™ Evolution™ 220 UV-Visible Spectrophotometer and integrating sphere (Figure 1).

Evolution 220 UV-Visible Spectrophotometer (left), and sample compartment integrating sphere accessory (right)

Figure 1. Evolution 220 UV-Visible Spectrophotometer (left), and sample compartment integrating sphere accessory (right). Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

The cover deliberately included significant quantities of particulates, designed to efficiently scatter any transmitted light. Transmittance was measured with an integrating sphere.

Pieces of covers from both functional and failing devices were placed at the sphere’s transmittance port, with spectra collected from 220 to 800 nm (Figure 2).

Diffuse Transmittance UV-Visible spectra of the failed cover (blue) and good cover (red), collected with an Evolution 220 UV-Visible Spectrophotometer and integrating sphere accessory.

Figure 2. Diffuse Transmittance UV-Visible spectra of the failed cover (blue) and good cover (red), collected with an Evolution 220 UV-Visible Spectrophotometer and integrating sphere accessory. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

While no transmittance could be measured through the functional cover, a significant transmittance was measured through the failing cover, comprising a visible part of the spectrum greater than 7% T.

These findings explained the poor performance stemming from the light leak when the device was operated under ambient conditions. However, these findings alone were not enough to identify the root cause.

Thermogravimetric Analysis (TGA)

A TA Instruments™ thermogravimetric analyzer was used to measure small pieces of both covers to determine bulk composition (Figure 3).1

Thermogravimetric analysis weight loss curves for the good cover (red) and failed cover (blue), showing that the good cover has significantly higher inorganic content than the failed cover.

Figure 3. Thermogravimetric analysis weight loss curves for the good cover (red) and failed cover (blue), showing that the good cover has significantly higher inorganic content than the failed cover. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

Samples were heated from ambient to 650 °C at a rate of 20 °C per minute under N2 purge prior to being cooled to 550 °C. They were then heated again to 1000 °C at a rate of 20 °C per minute with air purge.

The application of the initial heating ramp under nitrogen pyrolyzes the covers’ organic component while the final temperature ramp in air burns the remaining organic components. This process results in only oxides of the inorganic content being left behind.

The two covers’ organic decomposition profiles were almost identical, confirming that both had the same plastic composition.

The functional cover was found to contain a residual inorganic component representing 5.4% by weight, however, while the failed cover contained an inorganic component of 2.2% by weight. This significant difference in inorganic filler contents provided a strong clue as to the light leak’s source.

Infrared Analysis

The integrated diamond iS50 ATR on a Thermo Scientific™ Nicolet™ iS50 FTIR Spectrometer was used to acquire infrared spectra of small pieces of each cover (Figure 4).

Nicolet iS50 FT-IR spectrometer with built-in diamond iS50 ATR, iS50 ABX Automated Beamsplitter exchanger, and sample compartment iS50 Raman accessory.

Figure 4. Nicolet iS50 FT-IR spectrometer with built-in diamond iS50 ATR, iS50 ABX Automated Beamsplitter exchanger, and sample compartment iS50 Raman accessory. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

The Nicolet iS50’s built-in iS50 ATR features a dedicated detector which enables the collection of combined mid- and far-IR ATR spectra down to 100 cm-1 – factors which enabled the easy measurement and identification of inorganic fillers in the plastic parts.

The robust combination of these capabilities and the iS50 ABX Automated Beamsplitter exchanger on the Nicolet iS50 Spectrometer meant that both mid- and far-IR spectra could be automatically collected prior to being stitched together with a Thermo Scientific™ OMNIC™ Macros\Pro™ Visual Basic Program.

This provides a single spectrum of a sample from 4000 to 100 cm-1.2

ATR spectra of the plastic parts (Figure 5) were corrected using the OMNIC software’s advanced ATR correction algorithm.3 This algorithm considers and accounts for relative intensity changes resulting from sample penetration depth as a function of wavelength.

Advance ATR corrected infrared ATR spectra of the good plastic cover (top), failed plastic cover (middle) and difference spectra between the two (bottom).

Figure 5. Advance ATR corrected infrared ATR spectra of the good plastic cover (top), failed plastic cover (middle) and difference spectra between the two (bottom). Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

It also accounts for peak shifts in the infrared spectra as a result of the index of refraction differences between the ATR crystal and sample.

A review of infrared spectra from the two plastic pieces reveals a similar polymer composition, but the original plastic part exhibits an elevated baseline below 800 cm-1 and a sharp peak at 360 cm-1 (Figure 6) that are very weak or even absent in the spectrum from the replacement part.

Overlay of the advanced ATR corrected spectra of the good cover (blue) and failed cover (red), over the spectral region from 940 to 100 cm-1. Note the elevated baseline and the absorbance band at 360 cm-1 in the spectrum of the good cover that are absent or  highly reduced in the spectrum of the failed cover

Figure 6. Overlay of the advanced ATR corrected spectra of the good cover (blue) and failed cover (red), over the spectral region from 940 to 100 cm-1. Note the elevated baseline and the absorbance band at 360 cm-1 in the spectrum of the good cover that are absent or highly reduced in the spectrum of the failed cover. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

While the peak at 360 cm-1 is below the range of a standard mid-IR spectrometer equipped with a KBr beamsplitter, the use of the iS50 ABX with a solid substrate far-IR beamsplitter allows access to the far-IR range without compromising its high performance across the complete range.

The use of spectral subtraction emphasizes additional differences between the spectra. The difference spectrum (Figure 5, bottom) exhibits subtle peak shifts in the polymer bands, suggesting the presence of polymer composition difference between the two parts.

While this difference is expected when comparing plastic parts made by different manufacturers, a substantial spectral difference should also be noted below 800 cm-1.

Performing a library search of the difference spectrum and evaluating this against a forensic library of automobile fillers and paint pigments4 (Figure 7) returned a match for rutile – a crystalline form of titanium dioxide. This suggested a notable formulation difference between the two covers.

FT-Raman difference spectrum between the good and failed covers (blue), and top match from a library search against a forensic automobile paint pigment and fillers library (red), identifying a higher concentration of rutile (titanium dioxide) in the good cover.

Figure 7. FT-Raman difference spectrum between the good and failed covers (blue), and top match from a library search against a forensic automobile paint pigment and fillers library (red), identifying a higher concentration of rutile (titanium dioxide) in the good cover. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

FT-Raman Analysis

In order to verify the findings achieved via infrared analysis, the two samples were also analyzed via the iS50 Raman sample compartment FT-Raman accessory on the Nicolet iS50 Spectrometer (Figure 4).

The iS50 Raman accessory simply snaps into the Nicolet iS50 FTIR Spectrometer’s sample compartment without the use of an external module - a typical requirement for other FTIR spectrometer systems.

The iS50 Raman accessory facilitates the straightforward collection of Raman spectra thanks to the near-infrared beamsplitter and InGaAs detector mounted inside the spectrometer.

Figure 8 shows FT-Raman spectra acquired from the functional and failed covers and their spectral difference spectrum.

FT-Raman spectra of the good cover (top), failed cover (middle), and subtraction result between the two (bottom)

Figure 8. FT-Raman spectra of the good cover (top), failed cover (middle), and subtraction result between the two (bottom). Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

FT-Raman spectroscopy also enables the collection of spectra into the far-IR region, confidently complementing the capability of the Nicolet iS50 FTIR Spectrometer with the built-in iS50 ATR and ABX in terms of access to this region.

The two spectra were found to be very similar using this approach, again highlighting a similar polymer composition. Small differences were again visible in the spectra below 800 cm-1, as highlighted by the difference spectrum.

A library search of the difference spectrum against a minerals Raman library5 (Figure 9) also identified the difference between the two plastic parts as rutile, verifying the findings of the infrared analysis.

FT-Raman difference spectrum between the good and failed covers (top), and top library search result against a minerals Raman library (bottom), identifying a higher concentration of rutile (titanium dioxide) in the good cover.

Figure 9. FT-Raman difference spectrum between the good and failed covers (top), and top library search result against a minerals Raman library (bottom), identifying a higher concentration of rutile (titanium dioxide) in the good cover. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

Summary and Conclusion

Erroneous measurements for low light level measurements were caused by ambient light leaking into the device. It was determined, via diffuse transmission measurement of the parts by UV-visible spectroscopy, that the failed cover did not meet the maximum transmittance specification.

Further investigation via thermogravimetric analysis showed that the composition of the original cover contained approximately 3% more, by weight, of an inorganic filler in comparison to the replacement cover.

Infrared ATR analysis over the mid and far-IR spectral regions then revealed that the original cover contained much higher levels of rutile (titanium dioxide) than the replacement cover - results which were then confirmed via FT-Raman spectroscopy.

The example investigation presented here clearly illustrates the importance of employing an appropriate range of tools in root cause analysis.

Many of the tools used in this study are available on the Nicolet iS50 FTIR Spectrometer. The instrument is able to acquire multi-range spectra with no compromise in quality or speed, thanks to its built-in iS50 ATR and iS50 Raman accessories.

The robust combination of analyses provided by the Thermo Scientific UV-Vis and FTIR instruments, and the addition of thermogravimetric analysis, were decisive in ascertaining the root cause failure of the plastic cover in this example.

References

  1. Thermogravimetric results provided by Jeff Jansen, The Madison Group, 2615 Research Park Drive, Madison, WI, 53711.
  2. Mid-Far ATR iS50 collection program available upon request. Requires Nicolet iS50 FTIR Spectrometer configured with built-in diamond iS50 ATR, and ABX Automated Beamsplitter exchanger with KBr and solid substrate beamsplitter.
  3. Thermo Scientific Application Note 50581, Advanced ATR Correction Algorithm.
  4. An Infrared Spectral Library of Automobile Paint Pigments (4000–250 cm-1), developed by Dr. Edward H. Suzuki at the Washington State Police Crime Laboratory, downloadable from the SWGMAT.org website
  5. Downs R T (2006) The RRUFF Project: an integrated study of the chemistry, crystallography, Raman and infrared spectroscopy of minerals. Program and Abstracts of the 19th General Meeting of the International Mineralogical Association in Kobe, Japan. O03-13 Minerals 514 Raman Library.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

For more information on this source, please visit Thermo Fisher Scientific – Materials & Structural Analysis.

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